Carbonfund.org lets you estimate how much carbon your household creates, and then purchase the rights to produce that much carbon. Carbon you purchase is carbon that can no longer be produced by private corporations.

How much does carbon cost? Well, it trades on a market, therefore individual economic agents with competing demands and factor endowments set the price. According to Carbonfund.org's estimates of my household consumption, carbon is going for about $6/ton, or about as much as a nice cheese steak sub.